This study was designed to investigate the effects of growth hormone (GH) and sex steroid on the process of acquiring bone mass in childhood and adolescence. [Patients and Methods] The GH study on growing bone was performed with GH deficiency. The effect of estradiol on bone was investigated with 63 normal control children and 96 patients with short stature. [Results] The effect of GH on bone was initially observed in the increment of size predominantly, then bone mineral content from the second and the third year in females and in males, respectively. The new sensitive assay system for estradiol was used for the low serum concentration during childhood. Age-related changes of serum estradiol were clearly demonstrated with relatively high value during infancy, the lowest value at prepubertal age, and the increase at puertal age. The serum concentration of estradiol significantly correlated with volumetrically adjusted lumber BMD and Zscore of BMD. This correlation was observed in both patients with GH deficiency and without GH deficiency. [Discussion] GH stimulates bone metabolism and increased both size and bone mineral content. The action of estradiol on growing bone remains unclear. Ultrasensitive assay system for estradiol may become a powerful tool for the investigation. From our preliminary study, even at low serum concentration in males, estradiol may affect bone density in childhood.